1. Field of the Invention
The present invention is directed to an austenitic iron-base alloy containing nickel and chromium which has been solution-strengthened as well as precipitation-hardened and finds use both for fuel cladding and as a duct material in liquid metal fast breeder reactors. Since the alloys of the present invention are utilized as fuel cladding as well as a duct material it will be apparent that mechanical properties at elevated temperatures are of great importance. In addition, since the alloys will be under the constant influence of irradiation during operation as a fuel cladding material within a liquid metal fast breeder reactor, it becomes apparent that heavy emphasis must be placed on the low swelling characteristics of the alloy or at least having known swelling tendencies within given constraints.
In order to achieve these ends it has been found advantageous to control the chemistry of the alloying components such that upon the requisite precipitation-hardening, the matrix composition of the remaining alloy will be balanced in such a way as to provide for the low swelling tendencies without compromising the mechanical strength which attributes are necessary within the contemplated field of use of the subject composition.
2. Description of the Prior Art
For over 20 years the commercial composition known as A-286 has been utilized extensively for operation at elevated temperatures. A-286 is a wrought alloy containing nominally about 0.08% carbon, about 1.25% manganese, about 1.0% silicon, about 14.75% chromium, about 26% nickel, about 1.25% molybdenum, about 2.10% titanium, about 0.35% aluminum, about 0.25% vanadium, about 0.005% boron, and the balance iron with incidental impurities. This composition of matter in general terms has been described in such patents as U.S. Pat. Nos. 2,519,406 to Scott et al, 2,641,540 to Mohling et al, 3,199,978 to Brown et al, and 3,212,884 to Soler et al. An examination of all of these patents makes it clear beyond equivocation that the primary concern with the inventors is to obtain the requisite strength at elevated temperatures commensurate with sufficient ductility that the steels or austenitic alloy compositions would be useful for example in gas turbine parts which are subject to dynamic stresses. In none of these patents was any consideration given to controlling the swelling tendency of these alloys especially where the same are subject to the influence of irradiation over extended periods of time at elevated temperatures.
While commercial A-286 has the requisite strength for the intended temperature range of operation to which the present composition of matter is directed, nonetheless there is no suggestion as to how to control the swelling tendencies of such alloys. Consequently the candidate material which has been originally selected for fuel cladding and for duct work applications in the liquid metal fast breeder reactor has been a 20% cold worked stainless steel of the AISI Type 316 composition. Upon investigation of the swelling tendency of AISI Type 316 especially as predicted by nickel ion bombardment using a Van de Graaf apparatus, it becomes clear that the Type 316 candidate material has an extreme swelling problem in comparison with commercial A-286.
With these considerations in mind, it has been postulated that account must be taken of the solid solution strengthening components and the precipitation-hardening components together their effect upon the matrix chemistry since it is believed that the swelling is due in major part to the control of the matrix chemistry after having due regard to the various precipitation reactions which take place. In this respect, there must be a nickel and chromium trade off in the base chemistry and it would appear that the silicon and boron contents also function to aid in controlling swelling, yet these latter two elements are a primary requisite for the attainment of a portion of the mechanical property prerequisite such as ductility at elevated temperatures. Within this realm the substitution solutes such as molybdenum, titanium and aluminum, must be considered both for their influence on the swelling characteristics as well as their function on the mechanical properties.
It has been found that the gamma-prime precipitate, which is the fundamental hardening and strengthening mechanism of the subject composition, appears to be insensitive to the degree of swelling which the alloy undergoes. Thus, it is with this thought in mind that it is necessary to minimize the amount of nickel and chromium which can be utilized for the proper control of swelling in these alloys. Consequently, emphasis can be placed upon the size of the gamma-prime and its distribution within the grains so as to obtain enhanced mechanical properites without detrimentally affecting the swelling tendency of the overall chemical composition.